How to control light spread in a volume?
You'll be better off with a Spot or Sun lamp.
Sharper shadows can be achieved by decreasing Volumetrics density.

Spot lamp


for straight rays decrease the spot shape Angle value
for harder shadows increase power and Blend value


Sun lamp


keeps rays perfectly straight
for harder shadows increase strength and decrease angle


Example of a Spot light with a narrow cone (8°).


Example of a sun lamp with perfectly parallel rays.